1. Field of the Invention
The invention relates generally to a current-perpendicular-to-the-plane (CPP) magnetoresistive (MR) sensor that operates with the sense current directed perpendicularly to the planes of the layers making up the sensor stack, and more particularly to a CPP MR sensor with an improved hard magnet biasing structure for longitudinally biasing the sensor free layer.
2. Background of the Invention
One type of conventional magnetoresistive (MR) sensor used as the read head in magnetic recording disk drives is a “spin-valve” sensor based on the giant magnetoresistance (GMR) effect. A GMR spin-valve sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu). One ferromagnetic layer adjacent the spacer layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and is referred to as the reference layer. The other ferromagnetic layer adjacent the spacer layer has its magnetization direction free to rotate in the presence of an external magnetic field and is referred to as the free layer. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the reference-layer magnetization due to the presence of an external magnetic field is detectable as a change in electrical resistance. If the sense current is directed perpendicularly through the planes of the layers in the sensor stack, the sensor is referred to as a current-perpendicular-to-the-plane (CPP) sensor.
In addition to CPP-GMR read heads, another type of CPP MR sensor is a magnetic tunnel junction sensor, also called a tunneling MR or TMR sensor, in which the nonmagnetic spacer layer is a very thin nonmagnetic tunnel barrier layer. In a CPP-TMR sensor the tunneling current perpendicularly through the layers depends on the relative orientation of the magnetizations in the two ferromagnetic layers. In a CPP-GMR read head the nonmagnetic spacer layer is formed of an electrically conductive material, typically a metal such as Cu. In a CPP-TMR read head the nonmagnetic spacer layer is formed of an electrically insulating material, such as TiO2, MgO or Al2O3.
The sensor stack in a CPP MR read head is located between two shields of magnetically permeable material that shield the read head from recorded data bits on the disk that are neighboring the data bit being read. The sensor stack has an edge that faces the disk with a width referred to as the track width (TW). The sensor stack has a back edge recessed from the edge that faces the disk, with the dimension from the disk-facing edge to the back edge referred to as the stripe height (SH). The sensor stack is generally surrounded at the TW edges and back edge by insulating material.
A layer of hard or high-coercivity ferromagnetic material is used as a “hard bias” layer to stabilize the magnetization of the free layer longitudinally via magneto-static coupling. The hard bias layer is deposited as an abutting junction onto an insulating layer on each side of the TW edges of the sensor. The hard bias layer is required to exhibit a generally in-plane magnetization direction with high anisotropy (Ku) and thus high coercivity (Hc) to provide a stable longitudinal bias that maintains a single domain state in the free layer so that the free layer will be stable against all reasonable perturbations while the sensor maintains relatively high signal sensitivity. The hard bias layer must have sufficient in-plane remanent magnetization (Mr), which may also be expressed as Mr·t since Mr is dependent on the thickness (t) of the hard bias layer. Mr·t must be high enough to assure a single magnetic domain in the free layer but not so high as to prevent the magnetic field in the free layer from rotating under the influence of the magnetic fields from the recorded data bits. High Mr·t is important because it determines the total flux that emanates from the hard bias layer towards the free layer for a given SH. As t decreases with smaller shield-to-shield spacing it is even more important to have high Mr. Moreover, to achieve a high Mr, a hard bias material with both a high saturation magnetization (Ms) and high squareness (S) is desired, i.e., S=Mr/Ms should approach 1.0.
The conventional hard bias layer is typically a CoPt or CoPtCr alloy with Hc typically less than about 2000 Oe. The desired magnetic properties are achieved by a seed layer or layers directly below the hard bias layer, such as seed layers of CrMo, CrTi and TiW alloys, and bilayers, including NiTa/CrMo and Ta/W bilayers. US 2010/0002336 A1 describes a hard biasing structure of an insulating layer, a MgO layer on the insulating layer, a Cr-containing seed layer on the MgO layer and a CoPt hard bias layer on the Cr-containing seed layer. The MgO seed layer has a crystalline structure that allows for the epitaxial growth of the Cr-containing seed layer.
More recently a chemically-ordered FePt alloy based on the L10 phase has been proposed as the hard bias layer. The FePt alloy as deposited is a face-centered-cubic (fcc) disordered alloy with relatively low Ku (approximately 105 erg/cm3), but after annealing is a chemically-ordered alloy with face-centered-tetragonal (fct) phase (L10 phase) yielding high Ku (approximately 107 erg/cm3). However, the chemically-ordered L10 phase FePt alloy requires high-temperature deposition (>400° C.) or high-temperature annealing (>500° C.), which are not compatible with current recording head fabrication processes. US 2009/027493 A1 describes a FePt hard bias layer with a Pt or Fe seed layer and a Pt or Fe capping layer, wherein the Pt or Fe in the seed and capping layer and the FePt in the hard bias layer interdiffuse during annealing, with the annealing temperature being about 250-350° C. U.S. Pat. No. 7,327,540 B2 describes a FePtCu hard bias layer, with Cu being present up to about 20 atomic percent, wherein chemical ordering occurs at an annealing temperature of about 260-300° C. However, alloying FePt with nonmagnetic elements such as Cu is undesirable because it reduces Ms and thus for a given S, it reduces Mr. U.S. Pat. No. 8,281,270 B1, issued Jul. 10, 2012 and assigned to the same assignee as this application, describes an FePt alloy hard magnet biasing layer on a Ir or Ru seed layer on an insulating layer of an aluminum oxide, a tantalum oxide, a titanium oxide, or a silicon nitride.
As the data density increases in magnetic recording disk drives, there is a requirement for a decrease in the read head dimensions, particularly the shield-to-shield spacing. Thus the hard biasing structure, i.e., the hard bias layer, its seed layer or layers, and the underlying insulating layer, should be as thin as possible while assuring magnetic stabilization of the free layer. For a given thickness of the hard biasing structure, a thinner insulating layer enables the use of a thicker hard bias layer, with correspondingly increased Mr·t.
What is needed is a CPP MR sensor with an improved hard magnet biasing structure that has a thin insulating layer and a seed layer that enables a chemically-ordered L10 phase FePt alloy hard bias layer with improved values of coercivity (Hc) and squareness (S).